Schools of medicine, veterinary medicine, public health, and dentistry have a shortage of academicians and researchers with clinical degrees (MD, DVM, DOS) who are qualified for teaching and research positions. This means that professional degree students (especially medical students) are less likely to be exposed to mentors that will encourage them to consider careers in research. The purpose of this program is to provide research training to physicians, veterinarians, and occasionally dentists to fill such positions. This program, which has been active for 25 years, provides research training in cancer biology, virology, and immunology through laboratory research experiences, didactic and practical courses, lecture programs, research seminars, research conferences, and journal clubs supervised by senior investigators. The program will allow and encourage, but not require, enrollment for a PhD or SD degree at the School of Public Health. It is anticipated that up to half of the fellows will select such a degree program, which will ordinarily require 3 years. The first 9 months involve formal coursework while the last 27 months involves research in a laboratory. Postdoctoral fellows who do not enroll in a SD program ordinarily spend 2 to 3 years in research training. A major strength of the program is the broad experience of the faculty supervisors and their proven commitment to clinically significant research. To attract the best candidates, several research programs are based in laboratories that address cancer research problems at the host and population level as well as at the molecular and cellular level. Major areas of research that are covered for training emphasis include: (1) the pathogenesis, epidemiology, and molecular biology of human retroviruses in development of leukemia and related diseases, (2) the pathogenesis of Epstein-Barr virus in lymphocyte immortalization and lymphoma development, (3) the role of oncogenes and related growth factors in human cancers, (4) mechanisms of cell transformation, (5) the molecular and cellular regulation of T cell recognition and (6) pathways of signal transduction and transcriptional control in lymphocytes and macrophages. The major training facilities are in the Depts. of Immunology and Infectious Diseases (Essex, Glimcher, Grusby) and Genetics and Complex Diseases (Yuan) at the Harvard School of Public Health, the Dana- Farber Cancer Institute (Cantor, DePinho, Dranoff, Livingston, Look, Rollins, Pai), the Brigham and Women's Hospital (Ho, Kieff), the Dept. of Pathology, Harvard Medical School (Howley), the Children's Hospital and Center for Blood Research (Alt).